1. Field of the Invention
The present invention relates to the field of electronic systems which require power at more than one voltage, and more particularly, to a DC to DC power converter utilizing switched capacitors and inductive current limiters to achieve high efficiency.
2. Description of the Related Art
The evolution of electronic devices from analog to digital circuitry has changed the power supply requirements for circuit components. Yesterday""s analog systems typically required a multitude of supply voltages, whereas today""s digital systems typically use power at only a few standard voltages such as +/xe2x88x925V or 3.3V. By reducing the number of supply voltages required, system designs benefit through lower cost, lighter weight, reduced volume and higher reliability due to the reduction in the number of power supply components.
In spite of this evolution in electronics, there are still a significant number of systems which require power at voltages in addition to the standard digital 5V or 3.3V levels. For example, systems which include data communication circuits often require negative voltages for compatibility with the Electronic Industries Association (ELA) RS232C interface, a popular interface for data communications, which requires voltage levels ranging from xe2x88x9225V to +25V. Furthermore, preamplifiers, required for many interfacing applications, often require a negative supply voltage in addition to a positive supply voltage which is greater than the standard digital voltage of 5V.
In order to satisfy the need for several different supply voltages in digital systems, DC to DC power converters are used to produce output voltages different from the standard input voltage. These converters are available in step-down configurations that reduce the voltage relative to the input, step-up configurations that increase the voltage relative to the input, and inverter configurations that reverse the polarity of the input voltage (e.g. +5V input results in xe2x88x925V output) and may be combined with either step-up or step-down configurations.
For computer system applications, DC to DC converters often operate in a low voltage, high-frequency switched environment. The explosive growth in the field of portable electronic devices, such as portable telephones, radio pagers, and notebook computers, has created a need for DC to DC converters which consume a minimum amount of power and take up as little space in the device as possible. Because batteries are the main power source for these portable devices, low-voltage circuitry is used to reduce power consumption and extend battery life. Battery energy is further saved by using a distributed power supply system with a switched controller which turns the individual converters on and off as they are needed.
Additional advantages with distributed systems can be achieved using controllers and converters which operate at a high frequency. Miniaturized electronics which typically operate at frequencies in the range of 100 MHz or more, such as semiconductor integrated-circuit devices, save significant amounts of space and weight in portable systems. These devices also can operate at low voltage and power consumption levels. In addition, improved thermal management and higher power densities as compared to conventional electronics makes integrated-circuit devices a natural choice for portable systems.
One circuit element frequently used in DC to DC converters is the inductor. Inductors are commonly used in the forward, buck (step-down) and boost (step-up) converters shown in FIGS. 1(a), 1(b) and 1(c), respectively (discussed below in more detail). Because conventional converters require inductors with an inductive value on the order of 1 micro-Henry (1xc3x9710xe2x88x926 H), the inductor used is typically bulky and expensive, and is attached externally to the semiconductor chip which contains the remainder of the converter circuit. Inductors capable of integration on a semiconductor chip are available, but only for lower inductance values. Therefore, there is a need for converter circuits that use low-inductance-value integrated inductors permitting inclusion of all converter components in a single semiconductor chip.
Another common approach for producing additional voltages, that is particularly suited for low-power applications, is the xe2x80x9ccharge pumpxe2x80x9d or xe2x80x9cflying capacitorxe2x80x9d voltage converter. Referring to FIG. 2, an inverting charge pump 50 operates by charging a xe2x80x9cpumpxe2x80x9d capacitor 58 during a clock""s first half-cycle, or xe2x80x9cpumping phase,xe2x80x9d to the level of a source voltage 54 via amplifier 56. During the clock""s second, non-overlapping half-cycle, or xe2x80x9ctransfer phase,xe2x80x9d the pump capacitor 58 is disconnected from the source 54 and connected, with its polarity switched, to a second xe2x80x9creservoirxe2x80x9d capacitor 68, thereby xe2x80x9cpumpingxe2x80x9d charge to the reservoir capacitor 68 and providing an output VBB which is approximately the negative of the input voltage.
With a minor rearrangement of the pump""s switching elements, a step-up converter is produced. During the clock""s first half-cycle the pump capacitor is charged to the level of the source voltage. During the clock""s second half-cycle, the pump capacitor""s positive side is disconnected from the source, and its negative side, which had been connected to ground during the first half-cycle, is connected to the source. The positive side, now at twice the source voltage, is connected to the reservoir capacitor, thus charging it to twice the source voltage. This xe2x80x98doubledxe2x80x99 voltage at the reservoir capacitor is then used as a power supply to components requiring the doubled voltage.
Charge pumps are limited in their voltage ranges and ability to supply large currents. Large currents are required to reprogram electrically-erasable programmable read-only memory (EEPROM) arrays, making charge pumps unsuitable for these increasingly popular devices. Conventional forward, buck, and boost converters require large-inductance inductors and/or transformers which are difficult or impossible to fabricate on integrated circuits, increasing the size of the converter.
In addition to size, current and voltage ranges, efficiency is also an important aspect of DC to DC converter performance. All DC to DC converters will dissipate a portion of the input energy in the circuit components, for example some energy is dissipated as heat in each resistor. Greater component losses result in reduced efficiency of the converter. In general, greater current magnitudes over time in the circuit result in greater losses in circuit components and hence lesser efficiency. Also, the use of multiple clocks for switching transistors also dissipates energy and reduces efficiency.
Therefore, there is a desire and need for efficient DC to DC converters suitable for use in small portable electronic systems which operate at high frequency and are capable of producing a current output sufficient for EEPROM programming and a range of voltages sufficient to meet various system requirements.
The present invention provides a novel class of DC to DC converters based on switched capacitors suitable for use in portable electronic devices that offers improved efficiency, smaller size, and other advantages over conventional converters.
The above and other features and advantages of the invention are achieved by providing a DC to DC power converter circuit using switched capacitors where the switches are implemented by CMOS transistors or diodes on an integrated-circuit chip and using inductors to limit charging currents. The inductors can be fabricated directly on the CMOS integrated circuit or alternatively could be small inductors incorporated in the packaging. The high-frequency operation (100 MHz or greater) of the converter circuit permits the use of inductors with a low inductance value on the order of 100 nH (100xc3x9710xe2x88x929 Henrys) capable of fabrication directly on an integrated-circuit (IC) chip. The use of CMOS integrated components allows the entire converter to be formed on a single IC chip, saving significant space within the portable system.
Although the limit on charging current imposed by the inductor improves the efficiency of the converter by avoiding certain energy losses in the charging capacitor, the output current of the converter is not so limited as in prior designs, allowing the provision of a high output current for EEPROM reprogramming.
The present invention also provides reduced fluctuations in the power supply output voltage (ripple voltage) when several circuits are used in parallel to charge a single capacitor.
Furthermore, an embodiment of the present invention allows some transistor switches to be replaced with diodes to simplify the circuit and improve efficiency by removing the necessity for multiple switching clocks.